Positive and negative load compensation is very desirable since it provides control of fluid flow to and from the fluid motor. This fluid flow is proportional to the displacement of the direction control spool from its neutral position, irrespective of the magnitude of the positive or negative loads being controlled. An example of such a system is shown in my U.S. Pat. No. 3,858,393 which issued June 7, 1975. This type of control suffers from one serious disadvantage. When using a cylinder as a fluid motor during control of a negative load, the cylinder can be subjected to excessive pressure at the cylinder outlet and to cavitation at the cylinder inlet. This system also is limited by the capacity of the pump, since in control of the negative load, the pump flow is being used thus limiting the ability of the pump to supply other system loads.
My U.S. Pat. No. 4,058,140 which issued Nov. 15, 1977 overcomes this drawback in part since, during control of a negative load, the system pump is isolated from the cylinder by the negative load compensation and the cylinder inlet flow is supplied from the pressurized exhaust system. Although this system is very efficient and increases the capability of the pump to perform work while a negative load is being controlled, it suffers from the disadvantage of comparatively low system response. This is especially prominent during the change of the system load from positive to negative.
The present invention is directed to overcoming one or more of the problems as set forth above.